Facies and origin of triassic evaporites from the Nagra boreholes, Northern Switzerland

The NAGRA boreholes Boettstein, Leuggern, Riniken, Schafisheim and Weiach and the NOK well Beznau cored the evaporitic succession of the Gipskeuper and of the Anhydritgruppe of the Middle Muschelkalk. These formations are relatively poorly exposed in Northern Switzerland. Detailed mineralogical, textural and sedimentological analysis of these cores enabled reconstruction of the diagenetic and depositional history of the Triassic evaporites. A complex sequence of evaporitic minerals (including calcite, dolomite, gypsum, anhydrite, halite), together with chert and talc, is interpreted to represent an eogenetic assemblage. Subsequently, during mesogenetic (burial) diagenesis, gypsum dehydrated to anhydrite. Later replacement of anhydrite by mega-quartz and dolomite is recorded by sulphate-bearing carbonate beds. Concentrations of sphalerite, fluorite and mega-quartz are also present throughout the studied sequence. Ultimately, following post-Tertiary uplift, meteoric water ingress into the shallow-buried Triassic sediments of the Beznau, Boettstein and Leuggern wells resulted in telogenetic hydration of anhydrite, causing fracturing of the host rock and extensive dissolution of the sulphates. Although most original sedimentary structures are still preserved, practically all the primary authigenic minerals have suffered subsequent diagenetic modification since eogenesis. (author) 35 figs., tabs., 252 refs